Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech. The electrical stimulation is implemented through a prosthetic device, called cochlear implant, which is implanted in the inner ear to restore partial hearing to profoundly deaf people.
Such cochlear implants generally employ an electrode array that is inserted in the cochlear duct, usually in the scala tympani. One or more electrodes of the array selectively stimulate different auditory nerves at different places in the cochlea based on the pitch of a received sound signal. Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. There are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. In the cochlea, sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to basilar direction. The electrode array is fitted to the patient to arrive at a mapping scheme such that electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals.
The position of each electrode is not very precise. That is, there are only a limited number of electrodes, e.g., numbering about 16 to 24 electrodes, spread along the length of the electrode array, inserted into one of the spiraling ducts of the cochlea. Hence, accurately mapping to a “place” within the cochlea can be difficult, as the mapping is limited by the resolution of the discretely placed electrodes.
In a conventional cochlear implant, an envelope is extracted in each channel, and the remaining information, i.e., fine structure, is discarded. Given the number of channels in current processors, information in the fine structure can be very important for hearing certain sounds, particularly music. In previous disclosures, methods for encoding the fine time structure have been proposed. In these methods, stimulation is presented on one or more virtual or physical channels that has been optimally selected based on the estimate of the fine structure in each analysis band. However, these methods do not take into account the recent findings that non-simultaneous stimulation of nearby physical or virtual electrodes is perceived as a single pitch. For example, if in one band, the desired stimulation location is that corresponding to 1000 Hz, and in an adjacent band the desired location corresponds to 1300 Hz, then if both are present, the subject might perceive a pitch corresponding to 1150 Hz. This is known as decreased resolution, and it has a negative impact on the performance of current implants that seek to improve hearing by increasing the number of electrodes or using virtual electrodes to increase the number places to stimulate on a cochlea.